The present invention relates to a phosphor of formula A2SiO5:B (A is Y or Gd, and B is Ce or Tb), e.g., Y2SiO5:Ce, that emits blue light, and more particularly to a phosphor of formula A2SiO5:B (A is Y or Gd, and B is Ce or Tb) employed as an emitting layer in a field emission display(FED) device which utilizes a field emission cathode(FEC) as an electron source or in a vacuum fluorescent display(VFD) device, and method for preparing said phosphor.
Field emission display(FED) devices which use a field emission type cathode as an electron source have recently been under development for practical use. Considered as candidate fluorescent materials that may satisfy the blue emission chromaticity and efficiency requirements of FED devices are an Y2SiO5:Ce phosphor as well as a ZnS:Ag phosphor which is employed in a cathode ray tube (CRT). ZnS-based phosphors currently in use are phosphors for the CRT, such as ZnS:Ag,Cl, and phosphors for the projection tube, such as ZnS:Ag,Al.
However, when used in an FED or VFD device, a ZnS-based phosphor is decomposed and evaporated by electron beams impinged thereon, causing the fouling and degradation of the emitting source. Therefore, ZnS-based phosphors cannot be used in FED or VFD devices.
To solve the aforementioned disadvantages of sulfide phosphors, Y2SiO5:Ce phosphors emitting blue light have been studied. An Y2SiO5:Ce phosphor has inferior luminous color and brightness, as compared with a sulfide phosphor, but its use as an emitting layer in FED devices has been studied primarily because it does not foul the emission source.
An Y2SiO5:Ce phosphor generally has an extremely short lifetime when used in VFD devices, whereas in FED devices, it has a better emission efficiency than in VFD devices, but still has a relatively short lifetime and also exhibits a sign of emitting source fouling.
To solve the problems mentioned above, the present inventors have suggested, in Japanese Patent Application No. 10-106332, a modified Y2SiO5:Ce phosphor whose surface is rich in yttrium and contains BaO as a surface-modifying agent.
When patterning a fluorescent layer with a luminous material in display devices, a slurry containing the luminous material is generally use in the coating process. The above-mentioned modified Y2SiO5:Ce phosphor, however, is difficult to use in such a patterning process, because BaO, present on the surface of the phosphor particles as a surface-modifying agent, dissolves in the slurry and causes the gelation a photosensitive agent therein.
Therefore, the present inventors have endeavored to develop a blue emission phosphor having improved lifetime characteristics and luminous efficiency that can be used in FED devices without the aforementioned problem.
Accordingly, it is an object of the present invention to provide an improved phosphor which has a prolonged lifetime and improved luminous efficiency when used as an emitting layer in FED devices.
In accordance with one aspect of the present invention, there is provided a phosphor of formula A2SiO5:B which is characterized by a surface elemental composition represented by (A+B)/Si ranging from 1.5 to 2.5, wherein A is Y or Gd, and B is Ce or Tb.
In accordance with another aspect of the present invention, there is provided a method for preparing said phosphor.